Dual spectral area optical pyrometers are well known in the art and are used extensively in jet engine diagnostic testing. Dual spectral area optical pyrometers have been developed in order to differentiate between reflected and emitted radiation received from a target turbine blade and compensate for the error in the observed temperature that the reflected radiation introduces. In U.S. Pat. No. 4,222,663, Gebhart, et al discloses a dual band (two color) optical pyrometer which comprises two separate pyrometers. Each pyrometer sees a different but overlapping component of the total spectral range of the light or radiation from the turbine blade.
Prior art dual spectral area optical pyrometers usually comprise an optical probe fitted into the bottom of the jet engine housing. Radiation from the rotating turbine blades is collected by the probe and provided to a fiber optic bundle which guides the optical beam to a detection module. A bifurcated fiber optic coupler is most often used to divide the optical beam into two optical beams, one of which passes through a conventional optical filter whose passband is selected to be less than the spectral width of the optical beam from the turbine blade. Photodetectors receive the optical beams and provide energy signals to a signal processor. The received energy signals are converted to linearized temperature signals. The signal processor provides corrected temperature signals.
Dual spectral areas optical pyrometers of the prior art have been limited to use as diagnostic tools for several reasons. The location of the optical probe is typically restricted to the bottom of the jet engine housing. As a result, dirt and moisture contaminate the optics used to gather the turbine blade radiation, requiring frequent cleaning. This problem is most accute during initial engine start-up and again during engine shutdown.
In addition, prior art detection modules are bulky and have optical components whose alignment can be deleteriously affected by mechanical vibration. The detection modules typically comprise a bifurcated fiber optic coupler which simply divides the target optical beam in two. A conventional optical filter is inserted in one of the two beams output therefrom. Consequently, the optical division is not spectral band sensitive, which wastes a portion of the radiation in each beam and limits the signal-to-noise ratio available.
The signal processors used in the prior art are hardware intensive because of the elaborate processing which must be performed in generating linearized temperature signals and in computing corrected temperature signals. Moreover, amplifiers must be selected to accommodate the large dynamic range of the energy signals at the expense of useful signal bandwidth. Consequently, the signal processors of the prior art are inappropriate for in-flight applications for engine control.